The present invention relates to a regenerative heater for heating and burning fine particles (hereinafter abbreviated to "particulates") such as carbon or the like captured by a filter from exhaust gas of a Diesel engine, and also relates to a particulate trap for a Diesel engine (hereinafter abbreviated to "DPF") using such a regenerative heater.
Exhaust gas of a Diesel engine includes fine particles mainly consisting of carbon and called particulates. A DPF has been developed for removing these fine particles from the exhaust gas. The DPF is configured by the combination of a filter portion for capturing particulates and a regenerative heater for burning the captured particulates to thereby remove them.
Ceramic porous bodies or metal porous bodies are used for such capturing filters. Indeed the ceramic porous bodies have very minute voids so that they can capture particulates in exhaust gas surely, but since the voids are very minute, the loss of pressure of the exhaust gas is large and correspondingly a large capacity is required. Further, since the thermal conductivity of the ceramic porous bodies is poor, cracks, breaks, melting or the like are apt to be caused due to local heating. Particularly in regeneration, heat spots are apt to be caused by uneven heating hereby causing damaged to the filters. In the metal porous bodies, on the other hand, the void size can be controlled in comparison with the ceramic ones, and the thermal conductivity is superior, so that heat spots are hardly caused, and it is easy to keep the distribution of temperature uniform during regeneration.
As for the means for burning and removing particulates, there is a method to heat and burn particulates by conducting a current to an electric heater. Japanese Patent Unexamined Publication No. Hei-6-257422 proposes an arrangement in which different-diameter cylindrical filters are combined in multi-layers, and a plate-like regenerative heater is disposed among the filters in a position where the heater does not contact with the filters. FIG. 9 shows, as a conceptual diagram, the state where this means is incorporated in a DPF.
Exhaust gas of a Diesel engine flows into a casing 105 from a flange connected to an exhaust pipe in the direction of the left arrow A, and particulates are captured when the exhaust gas passes through the voids of cylindrical filters 102. A regenerative heater 103 is disposed between the cylindrical filters 102. An air inlet 106 for sucking oxygen in the atmosphere and an air outlet 107 may be provided for heating and burning the particulates efficiently. To supply oxygen more positively, there are also a method in which an air fan is provided, and a method in which oxygen in exhaust gas is used. The purified gas is released into the atmosphere in the direction of the left arrow B.
With such a configuration, efficient regeneration can be realized because the captured particulates are heated directly. However, on the side where exhaust gas flows into the filters, the air in the atmosphere which is at a low temperature may be mixed into the inside from the air inlet, while on the outlet side, the heat escapes as radiation heat through a shield holding the filters so that the temperature is difficult to reach a predetermined value. At the center portion, on the other hand, self-burning of the particulates takes part thereby increasing the temperature to an unnecessarily high degree.
In the case where a filter has a multi-layered structure, thermal capacity is different between an outer-cylinder side and an inner-cylinder side of the multi-layers. In addition, when the respective cylindrical electric heaters are viewed individually, the inside circular space surrounded by the electric heater has little flow of heat so that a constant temperature is kept, while the outside space has an active flow of heat to the outside of the DPF so as to be easily cooled. Therefore, in the filter constituted by a number of layers of cylinders, the central cylinder is apt to take a high temperature.
Uneven temperature distribution directly reduces the regenerative efficiency of the heater.